When do you use a neck sizing die instead of Full length die?

When you progress to the anal stage!

neck size
when you're loading cases that were fired in your own rifle and you don't like to trim brass and want to get max case life. Also as one of the things if you're going for absolute max accuracy. FL size on new cases, some hunting rifles if neck sizing leaves them too tight - or you can usually just bump the shoulder back a tiny bit. And on semi autos like ar15 most of the time.

If you want to ensure that 100% of the time your rounds will chamber on the first try, exactly when you need them to, full length size everything. Especially with semi autos and hunting rifles. The accuracy differential between full length and neck only sizing - for the vast majority of shooters - is negligible.

So if loading for 2 different gun in same caliber more than likely need to FL size.

Yes, most likely.

It is probably that the chambers in the two rifles will be slightly different. Brass fired in one gun that is only neck sized may or may not fit in the other rifle's chamber, and vice versa. FL sizing will allow you to use all of the brass in either rifle's chamber consistently.

Either that, or, if you simply think you must neck size for some reason, you will have to keep your brass separated by which rifle fired it, and adjust you neck sizing die for each group of cases.

At some point, you will have to bump the shoulder back on the cases regardless.

Reason for asking is I was getting ready to buy a new set of dies. Whether to buy just FL set or spend the extra and a set with the neck sizer die also.

NEVER!

I full length size everything, every time. And they all get a .003" bump on the shoulders. I want them to make all the velocity they can and I want them to chamber every time. I do not want to get a case stuck in a chamber after it has been fired. It's a terrible feeling to have to beat a bolt back with the palm of your hand.

I resemble that remark.

Sounds like FL.

See my response to your "Case Life" post. I full-length resize my brass for all the reasons stated above, except for my .300 WM, which has a long chamber and allows the brass to stretch excessively.

I got such good results from use of a Lee collet die with my 223, I now have that type die for all my calibers. For shoulder bumping I use Redding Body Dies when the cases get a little snug when they are chambered. This approach was reached after much use of FL dies, NK dies, and FL Bushing dies. And...no case lube is needed with the Lee Collet Dies, which speeds things up a good bit.

I am not a long range shooter or competition shooter, so what those guys say has higher value than what I have to offer, but the above gives me best accuracy.

I'll have to do a little more research into the lee collet dies.

...in both your hand & your mind.

I think I'm going to go FL and keep it simple.

I am also a fan of the Lee Collet dies. have some and just sent off for a couple custom ones for calibers they do not offer them in.

Why? Accuracy. Less case stretching resulting in longer case life with less trimming. I anneal and bump the shoulder back every 3rd or 4th loading and have Lapua cases with 10 loadings and still good to go.

amen.

While I cannot disagree with extended case life I think there may be a law of diminishing returns too. I have never neck sized. I have many rifles that shoot one hole groups using FL sized brass every time. Why go to the extra effort and expense if your gun will shoot accurately without?

FL sizing is fine, depending on how you do it. Depends a good bit on your chamber size. If you have a big chamber and FL size to the point you are pushing the shoulder way back, you will eventually have some case head separations. Just FL size to bump the shoulder back a bit, like Fireman does, and you are in fine shape.

If you aren't making hot loads, that Lee Collet Die is great, and case life will be relatively long. If you are loading hot, then you'll need the body die or FL die to occasionally shrink the case a bit.

NK sizing is starting to be a fading trend. You are not likely to find a nk sizing fan in any shooting comp from br to long range. The reason is reliability most use a bushing fl die to control nk tension and bump a minimal amount usually .001 for bolt guns. I've gone from fl to nk sizing back to fl length for reliability alone ,that and I couldn't shoot the difference.

I mentioned it about a year ago, but I took some new Nosler cases and loaded and shot them. I used several types of sizing dies, which I think are as follows:
RCBS FL sizing die
RCBS or Redding Neck sizing die
Redding FL type S Bushing Die
Lee Collet neck sizing die

I'm going on memory right now, so bear with me. The worse 10 shot groups were with the standard neck sizing die. Third place went to the standard FL sizing die. Second place was the Redding bushing die. Best accuracy was from the Lee Collet Die. This was all done with my Ruger Hawkeye, shooting 40 gr Nosler BT's, with H335 powder and CCI BR4 primers. And just me doing the shooting. I cleaned the rifle between each die change, which I guess caused me to shoot about 50 times between cleanings. The differences in accuracy weren't really large, though the standard neck sizing die was easily the worst. And after all that, I shot groups with mixed cases that had not been prepped, with Winchester, Remington, some Hornady, and some Nosler. Accuracy with them was not bad, though was slightly worse than the previous workups. All in all, for just hunting purposes I don't think most people would have noticed a difference.

But I did enough shooting to decide that the Lee Collet Die was easily the best value for the dollar. Easy to use. No case lube needed. Much faster to use overall, and the best accuracy (though only slightly). So I bought more of them. One was for my 260, but then I sold the 260 and bought a new one with a tighter chamber. Had about 100 rounds that would not chamber, so I got a body die and bumped the shoulder back a wee bit and the 'old' 260 ammo chambered and shot great.

I don't use standard neck dies any longer. I will be happy to sell them in 223, 260, 270, and maybe the one for the 220 Swift (which shoots great pretty much no matter how I size the cases).

Here is a good read on the various types of sizing and what it changes on the case.

http://riflemansjournal.blogspot.com/2010/06/basics-resizing-case-dimension-changes.html?m=1